Need help with serious DTMF difficulty

Doing something unusual. Placing outbound calls from both/either SIP/IAX and trying to detect inbound DTMF. This is a complete reversal of the norm where the originator of the call, also is generating the DTMF. Have tried combination of SIP/IAX and various service providers The results confusing needless to say as follows:

set dtmfMode=rfc2833 ==> DTMF is received correctly back from some called destinations and not from others.

set dtmfMode=inband ==> The phones reverse their roles. The ones that worked on rfc2833 don’t and the ones that did now don’t

Later in the day, reversed. Phones that worked inband now work rfc2833 and wont work inband and vice versa.

Later in the day again. No phones work.

This has been going on for days and has similar attributes with dtmfmode=auto.

On an OUTBOUND call, how does SIP or IAX know what detection mechanism to use for INBOUND dtmf? How does the egressing end office/gateway equipment determine the correct mechanism for DTMF detection on the REVERSE channel?

Has anybody run into this and have any idea what the solution is?